A Flow Controlled Virtual Connection (FCVC) protocol for use in a distributed switching architecture is presently known in the art, and is briefly discussed below with reference to FIG. 1. This protocol involves communication of status (buffer allocation and current state) on a per virtual connection, such as a virtual channel connection or virtual path connection, basis between upstream and downstream network elements to provide a "no cell loss" guarantee. A cell is the unit of data to be transmitted. Each cell requires a buffer to store it.
One example of this protocol involves a credit-based flow control system, where a number of connections exist within the same link with the necessary buffers established and flow control monitored on a per-connection basis. Buffer usage over a known time interval, the link round-trip time, is determined in order to calculate the per-connection bandwidth. A trade-off is established between maximum bandwidth and buffer allocation per connection. Such per-connection feedback and subsequent flow control at the transmitter avoids data loss from an inability of the downstream element to store data cells sent from the upstream element. The flow control protocol isolates each connection, ensuring lossless cell transmission for that connection. However, since buffers reserved for a first connection cannot be made available for (that is, shared with) a second connection without risking cell loss in the first connection, the cost of the potentially enormous number of cell buffers required for long-haul, high-bandwidth links, each supporting a large number of connections, quickly becomes of great significance.
Connection-level flow control results in a trade-off between update frequency and the realized bandwidth for the connection. High update frequency has the effect of minimizing situations in which a large number of receiver cell buffers are available, though the transmitter incorrectly believes the buffers to be unavailable. Thus it reduces the number of buffers that must be set aside for a connection. However, a high update frequency to control a traffic flow will require a high utilization of bandwidth (in the reverse direction) to supply the necessary flow control buffer update information where a large number of connections exist in the same link. Realizing that transmission systems are typically symmetrical with traffic flowing in both directions, and flow control buffer update information likewise flowing in both directions, it is readily apparent that a high update frequency is wasteful of the bandwidth of the link. On the other hand, using a lower update frequency to lower the high cost of this bandwidth loss in the link, in turn requires that more buffers be set aside for each connection. This trade-off can thus be restated as being between more efficient receiver cell buffer usage and a higher cell transmission rate. In practice, given a large number of connections in a given link, it turns out that any compromise results in both too high a cost for buffers and too much bandwidth wasted in the link.
Therefore, presently known cell transfer flow control protocols fail to provide for a minimized receiver cell buffer pool and a high link data transfer efficiency, while simultaneously maintaining the "no cell loss" guarantee on a per-connection basis when a plurality of connections exist in the same link.